An example of a known organic light emitting element, which is called organic electroluminescence element (organic EL element), is formed by stacking a transparent electrode as an anode, a hole transport layer, an organic light emitting layer, an electron injection layer, and an electrode as a cathode in this order on the surface of one side of a transparent substrate. When a voltage is applied between the anode and the cathode, electrons injected into the light emitting layer through the electron injection layer and holes injected into the light emitting layer through the hole transport layer are recombined in the light emitting layer to generate an excited state, thereby causing light emission. The light emitted by the light emitting layer is emitted outside the element through the transparent electrode and the transparent substrate.
An organic light emitting element has features, for example, that it is of spontaneous light emission, and shows light emission characteristics of relatively high efficiency, and further can emit light in various colors. For example, it is expected to be used as a light emitter in a display device of e.g. flat panel display, or as a light source for a backlight for a liquid crystal display apparatus, or for illumination, and so on. It has already been put to practical use in some. However, an organic light emitting element has a tradeoff between brightness and life, and has properties such that an increase in brightness to obtain a sharper image and brighter illumination light shortens its life.
A known organic light emitting element to solve the above-described problem is shown, for example, in Japanese Laid-open Patent Publication Hei 11-329748. In this organic light emitting element, a plurality of light emitting layers are provided between an anode and a cathode, and further a charge generation layer or an intermediate conducting layer which forms an equipotential surface is provided between respective ones of the light emitting layers.
As demonstrated in FIG. 16, an organic light emitting element 101 of this kind is formed by stacking a light-transmitting electrode 103 as an anode, a first light emitting layer 104, an intermediate conducting layer 105 forming an equipotential surface, a second light emitting layer 106, and a light reflective electrode 107 as a cathode in this order, respectively, on the surface of a transparent substrate 102. Note that although the light emitting layers 104, 106 respectively have a hole transport layer and an electron injection layer, their drawings are omitted in FIG. 16.
According to the structure described above, the plurality of light emitting layers 104, 106 are partitioned by the intermediate conducting layer 105, so that when a voltage is applied between the electrodes 103, 107, both light emitting layers 104, 106 are brought to a state in which they are essentially connected in series, thereby emitting light at the same time. As a result, the lights from the respective light emitting layers 104, 106 are put together, so that the organic light emitting element 101 can emit light with a higher brightness than a conventional organic light emitting element with a single light emitting layer, despite the tradeoff between brightness and life.
However, an organic light emitting element has problems such as reduction in light use efficiency as well as view angle dependence and film thickness dependence of light emission brightness and light emission spectrum, which occur more noticeably in an organic light emitting element with a plurality of light emitting layers as described above. These problems are caused e.g. by light interference effect or total reflection which causes a phenomenon of light confinement in high refractive index media such as the organic light emitting layer, the substrate and the electrodes. These in turn are caused e.g. by that the organic light emitting element is a thin film device which has a film thickness in an order of magnitude of an optical wavelength, and which has therein a reflective surface composed of e.g. a refractive index step or a metal surface, or which has a high refractive index medium to generate light.
The light interference effect, if used properly, makes it possible to increase color purity and control directivity, and is particularly useful for uses such as flat panel display. For example, according to Japanese Laid-open Patent Publication 2000-323277, the film formation of organic material layers including a light emitting layer is performed such that the optical path length between a light emitting layer and a light reflective electrode is set equal to an odd multiple of ¼ wavelength of wavelength λ of the emitted light, or such that the optical path length between a light emission position and a maximum refractive index step position is set equal to an even multiple of ¼ wavelength of the wavelength λ, making it possible to emphasize the light with the wavelength λ. In particular, it is known that the optical path length between a light emitting layer and a light reflective electrode has a significant influence on the light emission spectrum. Further, Japanese Laid-open Patent Publication 2003-272860 shows that light emission of highest efficiency can be obtained, and the shape of a light emission spectrum becomes thin, by allowing an optical film thickness from a light emission position of each of a plurality of light emitting layers to a light reflective electrode to be equal to an odd multiple of ¼ wavelength of wavelength λ of the emitted light.
However, the light emission brightness and the light emission color vary significantly with a variation in the film thickness of the organic light emitting element whose color purity and so on are modified by optimizing the film thickness of the element such as the optical path length between the light emitting layer and the light reflective electrode, and the optical path length between the light emitting layer and the maximum refractive index step position as described above. This means that the film thickness tolerance, when manufacturing the organic light emitting element, is small, resulting in a risk that the manufacturing efficiency may be reduced. In particular, in an organic light emitting element formed by stacking a plurality of light emitting layers, an equipotential surface forming layer, a charge generation layer, and so on as described above, a slight deviation in optical properties (anomaly in film thickness and refractive index) occurring in one layer has a profound influence on the optical positions of the other layers, so that a highly accurate film thickness control is required, resulting in a risk of a high cost.
Further, in the light emitting element shown in the above-described Japanese Laid-open Patent Publication 2003-272860, it is preferable from the viewpoint of efficiency that the optical path length between the light emitting layer and the light reflective electrode is set equal to an odd multiple (2n+1) [n=0, 1, 2 . . . ] of ¼ wavelength of wavelength λ of the emitted light. However, actually, the angular dependence of the brightness and spectrum increases with an increase in the value of n. More specifically, the film thickness of an organic light emitting element with a single light emitting layer is designed to correspond an optical path length which substantially corresponds to n=0, so that the light emission brightness and the light emission color do not necessarily vary significantly with a variation in the film thickness. However, in an organic light emitting element comprising a plurality of light emitting layers as described above, each of the light emitting layers is provided at a position corresponding to a (2n+1) [n=0, 1, 2 . . . ] multiple of ¼ wavelength of the wavelength λ of the emitted light. Accordingly, an increase in the number of layers causes a specific wavelength to be noticeably emphasized, causing a certain light emitting layer to emit light of a different spectrum from its intrinsic spectrum, and also causing its angular dependence to increase. Thus, the organic light emitting element comprising the plurality of light emitting layers as described above does not have desirable properties with respect to the light emission spectrum and the view angle dependence, although it can achieve higher current efficiency and quantum efficiency than the conventional type organic light emitting element.
On the other hand, in the organic light emitting element having the structure shown in FIG. 16, the respective ones of the plurality of light emitting layers are connected in series, and the same current is supplied to the respective light emitting layers, so that essentially it is not possible to individually control the light emission colors of the respective light emitting layers. By selectively designing predetermined respective light emitting layers when manufacturing an organic light emitting element, it is possible to obtain an organic light emitting element in which the respective light emitting layers emit light with individual colors. However, it is not possible to change once-determined light emission colors. If a plurality of light emitting layers which present respective colors of RGB are stacked, for example, then white color is obtained by combining the light emission colors. However, if the behaviors of light emission properties of the respective light emitting layers to the brightness are different, it may not be possible to obtain desired white light due to changes in the light emission color between brightnesses. In addition, if the lives of the plurality of light emitting layers are different, it may occur that the light emission color of light emitting layer with a short life gradually decreases, causing a color deviation. For example, if the organic light emitting element having the structure described above is used as a light emission source of a display, a deviation may occur in the color balance of a displayed light emission color. Further, if the organic light emitting element described above is used as a light source for illumination, the degradation is viewed and recognized as a color deviation, which is particularly undesirable.
Besides, Published Japanese Translation of PCT Application 2001-511296 proposes a stacked type organic light emitting element formed by stacking a plurality of light emitting layers having electrodes. This organic light emitting element has a structure such that a plurality of light emitting layers with independent or partially common electrodes are stacked, if necessary via insulating layers, and is usable for display use. However, the organic light emitting element with this structure has a short distance between the light emitting layers, so that it does not solve the problem of the light interference described above. In fact, Published Japanese Translation of PCT Application 2001-511296 proposes an element design in which the positions of the respective light emitting layers are set based on their light emission wavelengths on the premise of the existence of light interference, so as to enable light emission with high color purity. However, this element design also uses such film thickness as to cause the distance between a light emitting layer and a light reflective layer to emphasize light with a predetermined wavelength, so that particularly in the case of stacked light emitting layers, the problem of the angular dependence of the light emission wavelength is not solved.